1. Field of the Invention
The present invention relates to a radially acting track follower system with an optical sensor for an audio or video disk record player.
2. Brief Description of the Background of the Invention Including Prior Art
The radial error signal for the radial readjustment can be generated by two additional light spots disposed in tangential direction relative to the track as is taught in German Patent DE-PS No. 2,320,477 (corresponding to U.S. Pat. No. 4,547,872). They are disposed at an equal distance from the main light spot and their centers are on a line and they are staggered in radial direction oppositely to the center of the track such that in case of a radial deviation of the track under the light spots there are generated opposite effects at the two auxiliary light spots. The reflected intensities of the two auxiliary light spots are sensed separately from each other and from the reflected light intensity of the main spot with appropriate detection means.
It is known from U.S. Pat. No. 3,876,842 from the summary and from FIGS. 1 and 4 together with the respective description that also an adjustable phase grating or diffraction grating can be employed. Here three scanning spots (LB.sub.1, LB and LB.sub.2 of FIG. 1) disposed on a line are generated. The center scanning spot also is employed for reading the information and for controlling the focussing and the two outer scanning spots (LB.sub.1, LB.sub.2) are employed for controlling the track.
It is also known from the initially mentioned patent to generate a total of three spots with a phase grating, which is adjustable over a small angle constructed for the one-time adjustment according to conventional apparatus constructions in a plane vertical to the light beam bundle around a rotation center point, which is not the center of the phase grating, but which is outside of the phase grating. A required rotation is provided mechanically via a lever with an adjustment set screw accessible from the outside of the apparatus.
A similar adjustment provision is taught in the European Patent Application Laid Open No. 0,073,066. Here an optical scanner is constructed according to the multiple three beam principle (FIG. 5 and the respective description) and it is mentioned to achieve a certain desired angle setting of the two scanning spots (6a, 6b, 6c.sub.2 and 6a.sub.2, 6b, 6c) by a rotation of the diffraction grating plates (4a, 4b). Thereby the angles (.theta..sub.1 and .theta..sub.2) of the two three beam systems are adjusted.
In the context of the present invention a phase grating includes a diffraction grating, which is an optical device comprising an assembly of narrow slits or grooves, which produce a large number of beams that can interfere to produce spectra. Preferably the grating is a substantially transparent optical medium, however a reflecting mirror could also be used under changing transmission optical components and the light path shown in the present application into a reflection optical component and corresponding light path.
All these conventional optical scanners accoring to the three beam principle are associated with the following disadvantages:
1. They cannot be employed, where based on the construction principle only one single radial control is provided with a tilting lever, since a tangential error angle, depending on the just scanned track radius and associated with such control, results in such a shifting of the position of the auxiliary light spots relative to the track such that a maintenance of the track is not any longer possible.
2. In case of a radial linear system with coarse and fine track readjustment there results the difficulty of an unfavorbale radial shifting of the two auxiliary light spots relative to the track in case of only very small deviations in the guiding of the radial adjustment based on the different curvature of the track depending on the scanning radius. Therefore, the maintaining of the track becomes deficient or a loss of the track can occur.
It is further known from "Philips Technical Review, 40/82, page 154" that a radially acting control signal can be applied to the radial track follower system for the improvement of the guide following properties, where the control signal effects a radial swinging back and forth of the light spots at a frequency of 600 hertz and with an amplitude of 0.05 micrometers. The radial swinging motion of the scanning device results in a superposition of the light intensitites on the detection elements. Conclusions can be reached relating to a possible asymmetry of the optical system by corresponding sum formation, filtering and comparing with the original control signal and thus errors in the radial error signal can be determined and avoided. However, such a control provision is associated with the disadvantage, that the control signal is superposed to the scanned information signals, that is it is buried in the information spectrum contents. Therefore, it is not possible to employ a higher frequency for the control signal, since the energy components of the information signals initially increase with increasing frequency. However, it would be desirable to employ a frequency higher than for example 500 or 600 hertz, since only then a sufficient distance can be achieved relative to the frequency spectrum of the radial error signals.